Known optical transmission systems can be broadly categorized as direct detection, or coherent detection systems. In direct detection systems, at the receiver, the signal power is measured and therefore any phase and polarization information in the optical signal is ignored and lost. In coherent detection systems, the phase and/or polarization information is detected which enables the use of polarization and/or phase modulation as well as amplitude modulation, and so much greater information carrying capacity is possible, than in direct detection systems for a given optical signal to noise ratio. Direct detection systems have nevertheless dominated the market for long haul transmission systems due to their simplicity. In contrast coherent receivers require careful polarization alignment and phase tracking, which is difficult and can limit the cost/performance trade off. In typical systems, the polarization may change at rates up to kHz levels, while phase variations can be typically up to MHz levels.
An example of a coherent system intended to be insensitive to polarization and phase fluctuations is shown in Cheng et al, Journal Lightwave Technology Vol 7, No 2, 1989. The incoming optical signal is split into two polarizations (e.g. vertical and horizontal polarizations using a Wollaston prism). Each of these two optical signals are then combined with a common optical local oscillator using an optical 90 degree hybrid to give in-phase and quadrature waveforms for the two polarization states. On detection using a photodiode this results in four electrical signals corresponding to the in-phase and quadrature waveforms for the two polarizations.
Both coherent and direct detection systems are also limited in high capacity systems by distortions introduced by the optical path, mostly optical fiber. There are many such distortions, including nonlinearities, four wave mixing, and so on, but the principal ones are usually chromatic dispersion (CD) and polarization mode dispersion (PMD). Chromatic dispersion is usually approximately fixed with respect to time, but may drift over the life of the fiber, or undergo step changes if the optical path is altered, for example by a protection switching operation, or switching in a wavelength routed network. PMD can vary over periods of minutes, and so needs adaptable control. Many complex solutions have been tried to compensate for PMD and CD with limited success. Solutions which correct the distortion in the optical domain involve expensive optical components.
An example of an electronic compensator for a conventional 10 Gb/s optical transmission system is described in a press release of Aug. 12, 2002 by Santel Networks, of Newark, Calif. They claim that it provides a single solution for mitigating impairments from PMD and CD, which may otherwise limit the reach of optical systems, impair quality-of-service levels or prevent deployment of service on legacy fiber. However, any such compensator will have a limited performance since in conventional direct detection systems, the optical field is not fully recovered, for example the phase and polarization information is lost.